WO2022127223A1

WO2022127223A1 - Preparation method for polyolefin microporous diaphragm

Info

Publication number: WO2022127223A1
Application number: PCT/CN2021/118235
Authority: WO
Inventors: 程跃; 宫晓明; 彭锟; 虞少波; 庄志
Original assignee: 上海恩捷新材料科技有限公司
Priority date: 2020-12-15
Filing date: 2021-09-14
Publication date: 2022-06-23
Also published as: CN112592510B; CN112592510A

Abstract

The present invention relates to the field of battery diaphragms. Disclosed is a preparation method for a polyolefin microporous diaphragm. The preparation method comprises: making a cast sheet pass through a boiling-type pore-forming agent removal unit to remove a pore-forming agent; stretching, at least in one axial direction, the sheet from which the pore-forming agent has been removed, so as to obtain a base membrane; and then carrying out stretching and heat-setting on the base membrane to obtain a polyolefin microporous diaphragm. The temperature of the pore-forming agent/a pore-forming agent removal liquid is increased, and the viscosity and the surface tension thereof are reduced, so that the pore-forming agent/pore-forming agent removal liquid can more easily enter and exit and more quickly pass through micropores, and the pore-forming agent removal efficiency is thus improved; therefore, when the sheet is subjected to MD/SBS stretching, the friction force between the sheet and an apparatus is increased without the need to reform the apparatus, thereby avoiding slipping between the sheet and the apparatus due to insufficient friction force, reducing the phenomenon whereby micropore closing/pore mis-location occurs in the sheet due to slipping, effectively ensuring a straight-through property of the micropores, and reducing impedance; and a higher molecular weight/solid content/stretching ratio can be used, so that the mechanical property of a final product diaphragm is greatly improved.

Description

A kind of preparation method of polyolefin microporous membrane

technical field

The invention relates to the field of battery separators, in particular to a preparation method of a polyolefin microporous membrane.

Background technique

At present, lithium-ion batteries have been widely used in the field of power, but the accompanying safety issues have attracted great attention from the society. As the core component of the lithium battery safety guarantee, the PE microporous membrane can ensure the mechanical isolation of the two poles and ensure good permeability during the transmission of lithium ions. To this end, in addition to ensuring that the lithium battery has a high ion pass rate, the key is safety. Specifically, the PE microporous membrane itself needs not only high acupuncture strength and high tensile strength, so as to prevent the internal short circuit of the lithium ion battery caused by the contact between the two electrodes when the battery is abnormally damaged. It also needs to have high porosity and small pore size to ensure high liquid absorption and strong permeability.

Currently used in PE microporous membranes for lithium-ion batteries, wet asynchronous stretching process is often used. The preparation process includes: by raising the polyolefin and the pore-forming solvent to melt in an extruder. After passing through the T-die head, cool down to form a thin sheet. Stretching in the MD (along the line direction) direction is performed by post-heating. Then, it is stretched along the TD (vertical production line) direction, and without heating, after removing the pore-forming agent at 0-20° C., it is stretched again along at least one axial direction to obtain a PE microporous membrane.

When the prior art process enters the MD stretching process section, the friction force between the stretching roller is insufficient (due to the thermal phase separation, a large amount of self-lubricating pore-forming agent is precipitated on the surface, and the MD roller stretching will have slippage), The slippage leads to the phenomenon of multi-microporous closed-cell/porous dislocation in the sheet, with fewer straight holes and higher impedance, and it is impossible to realize the preparation of higher molecular weight, higher powder ratio formulation, and higher stretching ratio diaphragm.

SUMMARY OF THE INVENTION

In view of this, the present invention expects to provide a preparation method of a polyolefin microporous membrane, so as to realize the preparation of a higher molecular weight, a higher powder-to-material ratio formulation, and a higher stretch ratio separator.

In order to achieve the above object, the technical scheme of the present invention is achieved in this way:

The invention provides a preparation method of a polyolefin microporous membrane, which comprises the following steps in sequence:

(1) mixing and heating the polyolefin resin and the pore-forming agent to a molten kneading solution;

(2) extruding the kneading solution from the die, and cooling to form a casting sheet containing a porogen;

(3) passing the cast sheet through the boiling pore-forming agent removal unit to remove the pore-forming agent;

(4) stretching the cast sheet after removing the pore-forming agent along at least one axial direction to obtain a base film;

(5) stretching and shaping the base film along at least one axial direction again to obtain the polyolefin microporous film;

Wherein, the boiling pore former removal unit includes a tank body, a driving heat roller, a driven heat roller, and a pore former removal liquid; the driving heat roller and the driven heat roller pass through the heat conduction connected to the heating channel in the roller. The oil heat exchange station can achieve a heating temperature of 50°C to 140°C; the residual rate of the pore-forming agent of the polyolefin microporous film is lower than 0.05%.

Further, the pore-forming agent accounts for 40-50% of the total mass of the polyolefin resin and the pore-forming agent, and the kinematic viscosity at 60° C. is 5-200 mm ² /s.

Further, the pore-forming agent is selected from one or more of liquid paraffin, mineral oil, and soybean oil.

Further, the weight average molecular weight of the polyolefin resin is 4.0-8.0×10 ⁶ , and the polyolefin resin accounts for 50-60% of the total mass of the polyolefin resin and the pore-forming agent.

Further, the polyolefin resin is selected from one or more of polyethylene, polypropylene, polyisopropylene or polybutene.

Further, the polyolefin resin is polyethylene.

Further, the driving heat roller and the driven heat roller are heated by the hot oil flowing in the rollers.

Further, the pore-forming agent removal liquid is an organic solvent that is mutually miscible with the pore-forming agent.

Further, the pore-forming agent removal liquid is dichloromethane.

Further, the stretching in step (4) is asynchronous biaxial stretching or synchronous biaxial stretching.

Further, when the stretching in step (4) is asynchronous biaxial stretching, it is specifically stretched 10-35 times along the MD direction first, and then stretched 10-20 times along the TD direction.

Specifically, it is firstly stretched 15 to 25 times in the MD direction, and then stretched 10 to 15 times in the TD direction.

Further, when the stretching in step (4) is simultaneous biaxial stretching, the stretching ratio is 10-20 times.

Specifically, the draw ratio is 10 to 15 times.

Further, in step (5), the base film is stretched again along at least one axial direction with a stretching ratio of 2 to 4 times.

Further, the groove body is a sealed groove body, and the passage path of the polyolefin microporous sheet coming out of the casting machine is designed as an open part.

Further, the pore former removal liquid is located in the sealing tank; the position of the driving heat roller is higher than the liquid level of the pore former removal liquid; the driven heat roller is immersed in the pore former removal liquid middle.

The beneficial effects of the present invention are as follows:

1) The pore-forming agent boiling removal unit of the present invention increases the temperature of the pore-forming agent removal liquid, improves the exchange rate, and increases the removal efficiency of the successful agent by adding a driving hot roller and a driven hot roller;

2) By the preparation method of the present invention, the roughness of the polyolefin sheet is increased, so that when the MD roll surface/SBS clamp is stretched, the friction force between the roll surface/clamp and the polyolefin sheet increases, and it is not easy to slip/unclamp, so that a higher molecular weight (4.0 ～8.0×10 ⁶ ) and the formula with higher powder ratio (50%～60%) can be effectively stretched;

3) The diaphragm prepared by the method of the present invention has high porosity, which can ensure high liquid absorption rate and strong permeability;

4) The mechanical strength of the separator prepared by the method of the present invention is greatly improved, the impedance is reduced, and it will be safer in battery usage scenarios.

Description of drawings

FIG. 1 is a schematic diagram of a porogen removal unit according to an embodiment of the present invention;

Fig. 2 is the preparation process flow chart of prior art polyolefin microporous membrane;

Fig. 3 is the preparation process flow chart of the polyolefin microporous membrane of an embodiment of the present invention;

Fig. 4 is the preparation process flow chart of the polyolefin microporous membrane of another embodiment of the present invention;

Component label description

1. Drive the heat roller

2. Driven heat roller

3. Pore former removal liquid

4. Extrusion

5. Cool into pieces

6. MD stretching

7. TD stretching

8. Removal of pore formers

9. TD secondary stretching

10. SBS stretch.

Detailed ways

The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, but not to limit the present invention.

The specific embodiment of the present invention provides a preparation method of a polyolefin microporous membrane, and the method comprises the following steps in sequence:

Wherein, the boiling pore former removal unit includes a tank body, a driving heat roller 1, a driven heat roller 2, and a pore former removal liquid 3; the driving heat roller 1 and the driven heat roller 2 can be heated at a temperature of 50 ℃ ℃～140℃; the residual rate of the pore-forming agent of the polyolefin microporous membrane is less than 0.05%, preferably less than 0.02%, more preferably less than 0.01%, and most preferably the residual amount of the pore-forming agent is 0.

In this application, there is no particular limitation on the pore-forming agent used, as long as it can sufficiently dissolve the polyolefin, and the pore-forming agent can be, for example, but not limited to, one of liquid paraffin, mineral oil, and soybean oil. or more. Most preferably the pore former is liquid paraffin.

As a pore-forming agent, liquid paraffin can form a multi-layered oriented pore structure inside the porous substrate after being melt-kneaded and extracted with a polyolefin resin such as polyethylene resin, which greatly increases the successive pulling of the gelatinous membrane. stretch factor. The higher the draw ratio and the degree of crystallization, the higher the mechanical strength of the porous substrate. Therefore, as a pore former, liquid paraffin can improve the tensile strength and puncture strength of the porous film, so that the thinning of the porous film can be further realized.

In this application, the term "polyolefin" refers to a polymer obtained by the polymerization or copolymerization of one or more olefins, including but not limited to the polyolefin resin selected from polyethylene, polypropylene, polyisopropylene or polyolefin One or more of butenes. It is further preferred that the polyolefin resin is polyethylene.

Further, the driving heat roller 1 and the driven heat roller 2 are heated by the hot oil flowing in the rollers.

Further, the pore-forming agent removal liquid 3 is an organic solvent that is mutually soluble with the pore-forming agent. Preferably, the pore former removal liquid is dichloromethane.

As shown in FIG. 1, further, the pore-forming agent removing liquid 3 is located in the sealing tank; the position of the driving heat roller 1 is higher than the liquid level of the pore-forming agent removing liquid 3; the driven heat roller 2 is immersed in the pore former removal solution 3.

By introducing the heat transfer oil, the driving heat roller 1 and the driven heat roller 2 are heated to 50°C to 140°C. The method for heating the rollers at 7 hours is the same, which belongs to the conventional technical means of those skilled in the art, and does not need to be described in detail here. At this time, the polyolefin microporous sheet heated by the driven heat roller 1 and the driven heat roller 2 immersed in the pore former removing liquid 3 will heat the methylene chloride at a temperature of 0 to 10° C. to 30° C. under normal conditions. ~39.8°C.

In the process of heating, the liquid molecules in the pore-forming agent removal liquid 3 gain more kinetic energy due to heat transfer and are very active. The energy generated by these kinetic energy is enough to break free from the force between the liquid molecules, so its viscosity decreases; in addition , the increase in temperature makes the molecular motion or vibration speed up, so that the intermolecular repulsion increases. In order to achieve equilibrium again, the intermolecular distance will increase, and the gravitational and repulsive forces will decrease, so that the gravitational and repulsive forces will reach equilibrium again, thus causing the liquid surface tension. decline. Therefore, it will make it easier for the pore-forming agent removal liquid 3 to enter the micropores, improve the exchange rate, and increase the removal efficiency of the successful agent, and the residual rate of the pore-forming agent will be lower than 0.05%. Further, by adjusting the raw material formula of different porogen proportions, and different heating temperatures of the driving hot roller 1 and the driven hot roller 2, the removal efficiency of the successful agent is improved, and the residual rate of the porogen will be lower than 0.02% and lower than 0.01%. Even the residual amount of porogen is 0.

Further, the stretching in step (4) is asynchronous biaxial stretching (MD+TD) or synchronous biaxial stretching (SBS).

As shown in FIG. 3 , when the stretching in step (4) is asynchronous biaxial stretching, after the success agent is removed by the pore former removing unit of the present invention, the roughness of the polyolefin sheet increases, so that the MD roll surface is stretched 6, the friction between the roller surface and the polyolefin sheet increases, and it is not easy to slip. Thereby, the cast sheet with higher molecular weight (4.0～8.0×10 ⁶ ) and higher powder ratio (50%～60%) of the present invention can be stretched firstly by 10～35 times along the MD direction, and then stretched along the TD direction. Extend 10 to 20 times. More preferably, the stretching ratio is 15 to 25 times in the MD direction first, and then 10 to 15 times in the TD direction.

As shown in FIG. 4 , when the stretching in step (4) is synchronous biaxial stretching, after the success agent is removed by the pore former removing unit of the present invention, the roughness of the polyolefin sheet increases, so that the SBS clamp is stretched for 10 When the friction force between the clamp and the polyolefin sheet increases, it is not easy to disengage the clamp. Thus, the casting sheet of the present invention with higher molecular weight (4.0-8.0×10 ⁶ ) and higher powder ratio (50%-60%) can be stretched by 10-20 times. More preferably, the draw ratio is 10 to 15 times.

Further, in step (5), the stretching ratio at which the base film is stretched again along at least one axial direction is 2-4 times.

After the high-rate stretching by the above different methods, the orientation of the final product separator is increased, so that its mechanical strength (tensile strength and needle punch strength) is greatly enhanced, and the slippage is avoided to cause microporous closed cells in the sheet. The phenomenon of dislocation / hole dislocation, there are many through holes, more lithium ion channels with high through rate are created, and the impedance of the separator is reduced.

The present invention will be described in detail below by means of examples.

In the following examples and comparative examples, the film performance tests were performed according to the following methods:

Film thickness: Use a Marr thickness tester to measure the width of the finished product at intervals of 10cm in the longitudinal direction, and then obtain the average film thickness;

Air permeability value: At room temperature, use Wangyan type air permeability meter to set the time for 100cc gas to pass through the diaphragm, and measure the stable value after 5 seconds;

Porosity: Take a 100mm×100mm sample, weigh it with an electronic balance, and combine with the polyethylene density, and convert it according to the formula: (1-weight/sample area)/weight×0.957×100%;

Maximum pore size: measured by the bubble point method using nitrogen gas using a narrow aperture tester;

Tensile strength & elongation at break: use electronic universal material testing machine XJ830, cutting sample size: 15mm × 20cm, 200mm/min travel speed for measurement;

Needle punch strength: Use the electronic universal material testing machine XJ830, clamp the sample to be tested, and use the front end diameter of 1mm (0.5mmR) to measure at a travel speed of 50mm/min;

Thermal shrinkage rate: Use the high temperature test box Espec SEG-021H to place the 100mm×100mm microporous film at 110°C for 1 hour, and measure the length by the image measuring instrument XTY-5040, and count the lengths in the TD and MD directions before and after heat drying, using the formula : (before heat treatment - after heat treatment)/before heat treatment × 100% conversion;

Kinematic viscosity: use the kinematic viscosity tester DSY-004, set the measurement temperature to 60 °C, and measure the kinematic viscosity after 1 hour of stability;

Residual oil rate: cut a 10mm×10mm diaphragm sample, weigh it with an electronic balance, place pure water in an Ultrasonic Cleaner 1740T, and place a 500ml beaker with 300ml of pure dichloromethane, put in the sample, set the ultrasonic time to 60s, Then place it in a 105 ℃ oven to dry for 5 minutes, use an electronic balance to weigh the weight before and after cleaning, and use the formula: (weight before treatment - weight after treatment) / weight before treatment × 100% converted residual oil ratio;

Impedance: Use the battery chamber sampler to add sample, add electrolyte to the 2/3 scale of the battery chamber, use the Agilent data acquisition instrument KEYSIGHT 34972A to select the resistance test channel, click Run, and wait for the device to automatically analyze the data.

Example 1

First, put 50% polyethylene by mass (Mw is 8.0×10 ⁶ ) and 50% white oil into the extruder at a flow rate of 240kg/h and extrude it at 220°C and 100rpm through a T-shaped The die was extruded, and after being cooled by contact with a chill roll at a temperature of 35°C, a cast sheet was formed. Then enter the pore former removal unit, heat the driving heat roller 1 and the driven heat roller 2 to 140°C by heat transfer oil, and then the methylene chloride in the tank is heated to 39.8°C, and the pore former removal process is performed. The cast sheet from which the porogen was removed was stretched 10 times in the machine direction (MD) 6 at 120° C. using a stretching machine, followed by 10 times stretching in the width direction (TD) 7 at 100° C. At 120° C., 2-fold secondary TD stretching was performed to set 9, and the film was wound with a winding roll to obtain a polyolefin microporous film with a thickness of 14 μm.

Example 2

First, put 55% polyethylene (Mw is 6.0×10 ⁶ ) and 45% white oil by mass percentage into the extruder at a flow rate of 650kg/h (regulating the feeding amount to make the thickness of the diaphragm product consistent) into the extruder, Under the conditions of 220° C. and 100 rpm, it was extruded through a T-die, and after being contacted and cooled by a cold roll at a temperature of 35° C., a cast sheet was formed. Then enter the pore former removal unit, heat the driving heat roller 1 and the driven heat roller 2 to 100°C with heat transfer oil, and then the methylene chloride in the tank is heated to 35°C to carry out the pore former removal process. The cast sheet after removing the porogen was stretched 20 times in the machine direction (MD) 6 at 120° C. using a stretching machine, followed by 15 times stretching in the width direction (TD) 7 at 100° C. At 120° C., 2-fold secondary TD stretching was performed to set 9, and the film was wound with a winding roll to obtain a polyolefin microporous film with a thickness of 14 μm.

Example 3

First, put the mass percentage of 60% polyethylene (Mw is 4.0×10 ⁶ ) and 40% white oil into the extruder at a flow rate of 1000kg/h (regulating the feeding amount to make the thickness of the diaphragm product consistent) into the extruder, Under the conditions of 220° C. and 100 rpm, it was extruded through a T-die head, and after being contacted and cooled by a cold roll at a temperature of 35° C., a cast sheet was formed. Then enter the pore former removal unit, heat the driving heat roller 1 and the driven heat roller 2 to 50°C with heat transfer oil, and then the methylene chloride in the tank is heated to 30°C to perform the pore former removal process. The cast sheet from which the porogen was removed was stretched 35 times in the machine direction (MD) 6 at 120° C. using a stretching machine, followed by 20 times stretching in the width direction (TD) 7 at 100° C. At 120° C., 2-fold secondary TD stretching was performed to set 9, and the film was wound with a winding roll to obtain a polyolefin microporous film with a thickness of 14 μm.

Example 4

First, put 50% polyethylene by mass (Mw is 8.0×10 ⁶ ) and 50% white oil into the extruder at a flow rate of 240kg/h and extrude it at 220°C and 100rpm through a T-shaped The die was extruded, and after being cooled by contact with a chill roll at a temperature of 35°C, a cast sheet was formed. Then enter the pore former removal unit, heat the driving heat roller 1 and the driven heat roller 2 to 140°C by heat transfer oil, and then the methylene chloride in the tank is heated to 39.8°C, and the pore former removal process is performed. The cast sheet after removal of the porogen was stretched by 10 times at 120°C with simultaneous biaxial stretching (SBS) 10, and then 2 times at 120°C by TD stretching 9. The roll was wound up to obtain a polyolefin microporous film having a thickness of 14 μm.

Example 5

First, put 55% polyethylene by mass (Mw is 6.0×10 ⁶ ) and 45% white oil into the extruder at a flow rate of 490kg/h (regulating the feeding amount to make the thickness of the diaphragm product consistent), and extruding, Under the conditions of 220° C. and 100 rpm, it was extruded through a T-die head, and after being contacted and cooled by a cold roll at a temperature of 35° C., a cast sheet was formed. Then enter the pore former removal unit, heat the driving heat roller 1 and the driven heat roller 2 to 100°C with heat transfer oil, and then the methylene chloride in the tank is heated to 35°C to carry out the pore former removal process. The cast sheet after removing the porogen was stretched by 15 times at 120°C with simultaneous biaxial stretching (SBS) 10, and then 2 times at 120°C by secondary TD stretching 9. The roll was wound up to obtain a polyolefin microporous film having a thickness of 14 μm.

Example 6

First, put the mass percentage of 60% polyethylene (Mw is 4.0×10 ⁶ ) and 40% white oil into the extruder at the flow rate of 800kg/h (regulating the feeding amount to make the thickness of the diaphragm product consistent) into the extruder, Under the conditions of 220° C. and 100 rpm, it was extruded through a T-die head, and after being contacted and cooled by a cold roll at a temperature of 35° C., a cast sheet was formed. Then enter the pore former removal unit, heat the driving heat roller 1 and the driven heat roller 2 to 50°C with heat transfer oil, and then the methylene chloride in the tank is heated to 30°C to perform the pore former removal process. The cast sheet after removing the porogen was stretched by 20 times at 120°C with simultaneous biaxial stretching (SBS) 10 using a stretching machine, and then 2 times secondary TD stretching was performed at 120°C. The roll was wound up to obtain a polyolefin microporous film having a thickness of 14 μm.

Comparative Example 1

First, put 20% polyethylene by mass (Mw is 3.5×10 ⁶ ) and 80% white oil into the extruder at a flow rate of 600kg/h and extrude it at 180°C and 80rpm through the T-shaped The die was extruded, and after being cooled by contact with a chill roll at a temperature of 35°C, a cast sheet was formed. The cast sheet was stretched 9 times in the machine direction (MD) 6 at 110°C using a stretching machine, followed by 8 times stretching in the width direction (TD) 7 at 110°C, and then subjected to dichloride at 15°C. Extraction was performed in the methane tank to remove the pore-forming agent, and then 2-fold secondary TD stretching was performed at 120° C. for 9-setting, and it was wound with a winding roller to obtain a polyolefin microporous membrane with a thickness of 14 μm.

Comparative Example 2

First, put 20% polyethylene by mass (Mw is 3.5×10 ⁶ ) and 80% white oil into the extruder at a flow rate of 650kg/h and extrude it at 180°C and 80rpm through the T-shaped The die was extruded, and after being cooled by contact with a chill roll at a temperature of 35°C, a cast sheet was formed. The cast sheet was stretched 10 times along the machine direction (MD) 6 at 110°C using a stretching machine. At this time, the roller surface and the cast sheet slipped, and the cast sheet was deviated during travel and failed to enter the width of the cast sheet. In the stretching process in the direction (TD) 7, a separator product could not be formed.

Comparative Example 3

First, put 35% polyethylene by mass (Mw is 3.5×10 ⁶ ) and 65% white oil into the extruder at a flow rate of 340kg/h and extrude it at 180°C and 80rpm through the T-shaped The die was extruded, and after being cooled by contact with a chill roll at a temperature of 35°C, a cast sheet was formed. The cast sheet was stretched 9 times along the machine direction (MD) 6 at 110°C using a stretching machine. At this time, the roller surface and the cast sheet slipped, and the cast sheet was deviated while traveling and failed to enter the width of the cast sheet. In the stretching process in the direction (TD) 7, a separator product could not be formed.

Comparative Example 4

First, put 20% polyethylene by mass (Mw is 4.0×10 ⁶ ) and 80% white oil into the extruder at a flow rate of 600kg/h and extrude it at 180°C and 80rpm through the T-shaped The die was extruded, and after being cooled by contact with a chill roll at a temperature of 35°C, a cast sheet was formed. The cast sheet was stretched 9 times along the machine direction (MD) 6 at 110°C using a stretching machine. At this time, the roller surface and the cast sheet slipped, and the cast sheet was deviated during travel and failed to enter the width of the cast sheet. In the stretching process in the direction (TD) 7, a separator product could not be formed.

Comparative Example 5

First, put 20% polyethylene by mass (Mw is 3.5×10 ⁶ ) and 80% white oil into the extruder at a flow rate of 600kg/h and extrude it at 180°C and 80rpm through the T-shaped The die was extruded, and after being cooled by contact with a chill roll at a temperature of 35°C, a cast sheet was formed. The cast sheet after removing the porogen was stretched by 9 times at 110°C with simultaneous biaxial stretching (SBS) 10, and then 2 times at 130°C by secondary TD stretching 9. The roll was wound up to obtain a polyolefin microporous film having a thickness of 14 μm.

Comparative Example 6

First, put 20% polyethylene by mass (Mw is 3.5×10 ⁶ ) and 80% white oil into the extruder at a flow rate of 730kg/h and extrude it at 180°C and 80rpm through the T-shaped The die was extruded, and after being cooled by contact with a chill roll at a temperature of 35°C, a cast sheet was formed. The cast sheet after the pore former was removed was stretched 10 times by synchronous biaxial stretching (SBS) 10 at 110°C. At this time, the cast sheet was disengaged in the SBS fixture, and the next step could not be carried out. In the stretching process in the width direction (TD) 7, a separator product could not be formed.

Comparative Example 7

First, put 35% polyethylene by mass (Mw is 3.5×10 ⁶ ) and 80% white oil into the extruder at a flow rate of 340kg/h and extrude it at 180°C and 80rpm through a T-shaped The die was extruded, and after being cooled by contact with a chill roll at a temperature of 35°C, a cast sheet was formed. The cast sheet after removing the pore-forming agent was stretched by 9 times at 110°C with simultaneous biaxial stretching (SBS) 10. At this time, the cast sheet was disengaged in the SBS fixture, and the next step could not be carried out. In the stretching process in the width direction (TD) 7, a separator product could not be formed.

Comparative Example 8

First, put 20% polyethylene by mass (Mw is 4.0×10 ⁶ ) and 80% white oil into the extruder at a flow rate of 600kg/h and extrude it at 180°C and 80rpm through the T-shaped The die was extruded, and after being cooled by contact with a chill roll at a temperature of 35°C, a cast sheet was formed. The cast sheet after removing the pore-forming agent was stretched by 9 times at 110°C with simultaneous biaxial stretching (SBS) 10. At this time, the cast sheet was disengaged in the SBS fixture, and the next step could not be carried out. In the stretching process in the width direction (TD) 7, a separator product could not be formed.

Table 1 lists the performance test results of the separators of Examples 1 to 6 and Comparative Examples 1 and 5.

Table 1 Comparison of diaphragm performance tests of Examples 1 to 6 and Comparative Examples 1 and 5

It can be seen from the comparative examples 1-4 and 5-8 that:

When using asynchronous biaxial stretching (MD+TD), high molecular weight (Mw is 4.0×10 ⁶ ) or 6 times MD stretching with high powder ratio of 35% has slipped and the product cannot be produced. When (Mw is 3.5×10 ⁶ ) ) molecular weight and 20% polyethylene powder ratio can be stretched by 9 times MD, but the MD stretching operation of 10 times and larger cannot be performed, and the higher molecular weight (4.0-8.0×10 ⁶ ) of the present invention cannot be achieved. The high-magnification asynchronous stretching preparation method of the polyolefin sheet extruded from the formula raw material with a high polyethylene powder ratio (50% to 60%) cannot solve the technical problem.

When using synchronous biaxial stretching (SBS), high molecular weight (Mw is 4.0×10 ⁶ ) or 6 times SBS stretching with high powder ratio of 35% has been stripped and the product cannot be produced. When (Mw is 3.5×10 ⁶ ) When the molecular weight and the ratio of polyethylene powder are 20%, it can be stretched by 9 times of SBS, but the SBS stretching operation of 10 times and more cannot be carried out, and the higher molecular weight (4.0-8.0×10 ⁶ ) and higher molecular weight of the present invention cannot be realized. The high-magnification synchronous biaxial stretching preparation method of the polyolefin sheet extruded from the formula raw material with the polyethylene powder ratio (50% to 60%) cannot solve the technical problem.

It can be seen from the comparison of Examples 1 to 6 with Comparative Examples 1 and 5 that the diaphragms prepared by the preparation method of the present invention have slightly lower air permeability, higher porosity and smaller maximum pore size than those prepared by the prior art process. , low impedance, extremely high acupuncture strength, extremely high tensile strength, high elongation, slightly low thermal shrinkage, and extremely low residual oil rate, which can ensure high liquid absorption rate and low residual oil rate in battery usage scenarios. Strong permeability, and better security performance.

The above content related to common knowledge will not be described in detail (for example, it is a routine operation in the art to adjust the feeding amount to make the thickness of the diaphragm product consistent), which can be understood by those skilled in the art.

The above-mentioned embodiments merely illustrate the principles and effects of the present invention, but are not intended to limit the present invention. Anyone skilled in the art can make modifications or changes to the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those with ordinary knowledge in the technical field without departing from the spirit and technical idea disclosed in the present invention should still be covered by the claims of the present invention.

Claims

A method for preparing a polyolefin microporous membrane, wherein the method comprises the following steps in sequence:

(1) mixing and heating the polyolefin resin and the pore-forming agent to a molten kneading solution;

(2) extruding the kneading solution from the die, and cooling to form a casting sheet containing a porogen;

(3) passing the cast sheet through a pore-forming agent removal unit to remove the pore-forming agent;

(4) stretching the cast sheet after removing the pore-forming agent along at least one axial direction to obtain a base film;

(5) stretching and shaping the base film along at least one axial direction again to obtain the polyolefin microporous film;

Wherein, the pore former removal unit includes a tank body, a driving heat roller, a driven heat roller, and a pore former removal liquid; the driving heat roller and the driven heat roller can be heated at a temperature of 50°C to 140°C; The residual rate of the pore-forming agent of the olefin microporous membrane is less than 0.05%.
The method for preparing a polyolefin microporous film according to claim 1, wherein the pore-forming agent accounts for 40-50% of the total mass of the polyolefin resin and the pore-forming agent, and the kinematic viscosity at 60°C is 5-200 mm 2 /s.
The method for preparing a polyolefin microporous membrane according to claim 1, wherein the pore-forming agent is selected from one or more of liquid paraffin, mineral oil, and soybean oil.
The method for preparing a polyolefin microporous film according to claim 1, wherein the polyolefin resin has a weight average molecular weight of 4.0-8.0×10 6 , and the polyolefin resin accounts for the polyolefin resin and the pore-forming agent. 50 to 60% of the total mass.
The method for preparing a polyolefin microporous membrane according to claim 1, wherein the polyolefin resin is selected from one or more of polyethylene, polypropylene, polyisopropylene or polybutene.
The method for preparing a polyolefin microporous membrane according to claim 5, wherein the polyolefin resin is polyethylene.
The method for preparing a polyolefin microporous film according to claim 1, wherein the driving heat roller and the driven heat roller are heated by hot oil flowing in the rollers.
The method for preparing a polyolefin microporous membrane according to claim 1, wherein the pore-forming agent removal solution is an organic solvent that is mutually soluble with the pore-forming agent.
The method for preparing a polyolefin microporous membrane according to claim 8, wherein the pore-forming agent removal liquid is dichloromethane.
The method for preparing a polyolefin microporous membrane according to claim 1, wherein the stretching in step (4) is asynchronous biaxial stretching or synchronous biaxial stretching.
The method for preparing a polyolefin microporous film according to claim 10, characterized in that: when the stretching in step (4) is asynchronous biaxial stretching, specifically stretching in the MD direction by 10 to 35 times first, Then stretch 10 to 20 times in the TD direction.
The method for preparing a polyolefin microporous membrane according to claim 11, characterized in that: firstly, stretching 15-25 times along the MD direction, and then stretching 10-15 times along the TD direction.
The method for preparing a polyolefin microporous membrane according to claim 10, wherein when the stretching in step (4) is synchronous biaxial stretching, the stretching ratio is 10-20 times.
The method for producing a polyolefin microporous membrane according to claim 13, wherein the stretching ratio is 10 to 15 times.
The method for preparing a polyolefin microporous film according to claim 1, characterized in that: in step (5), the stretching ratio of the base film to be stretched again along at least one axial direction is 2-4 times.
The method for preparing a polyolefin microporous film according to claim 1, wherein the groove body is a sealed groove body, and the passage path of the polyolefin microporous sheet from the casting machine is designed as an open part.
The method for preparing a polyolefin microporous film according to claim 1, wherein the pore-forming agent removal liquid is located in the sealing groove; the position of the driving heat roller is higher than the pore-forming agent removal liquid surface; the driven hot roller is immersed in the pore former removal liquid.